Ever since Curiosity landed last summer, the rover has been undergoing systematic tests on all of its instruments and equipment. The last major hurdle in this process, using the rock drill, has now been achieved. The drill was saved until last because it is the most technically-challenging component on the rover.
Curiosity is the first Mars rover to include a rock drill. Its predecessors, Spirit and Opportunity, carried scrapers. The more recent Phoenix lander also contained a special scraper designed to expose surface ice The drill is a big step forward, however, because it allows scientists to peer deeper into rocks of interest. Just like on Earth, objects on the surface of Mars are affected by the environment. Winds and dust storms can erode them, oxidation turns them the familiar shade of red, and cosmic rays can alter the surface chemistry. Only by drilling beneath the surface of a rock can information about its original state be determined. Once a hole is drilled, the removed material can be transferred to other instruments for analysis.
The drill's operation was uncertain enough that NASA engineers opted for a series of escalating test holes before attempting a full-on bore. You can see one of these tests in the image above. The deeper of the two visible holes is 1.6 cm in diameter and 6.4 cm deep.
Now that Curiosity appears to be fully functional, it can begin to leave the immediate area of its landing and begin a journey across the surface of Mars.